In relation to dental work in general or in overall potential physical pain?
Dental work scale only: 7 to 8, depending on your dentist and the method used (modern methods are pretty excellent, but damn, it's still invasive stuff). To get to 10 in my book, they would need to totally botch the procedures and cause severe damages.
Overall potential pain: 3 if you are female and 5 if you are male (child birth weighs a lot here). Whether the canal is good or bad, there's worse pain to suffer and a canal lasts a pretty short while, in the grand scheme of things.
Now, I've had a few extractions recently and those were, surprisingly, not so bad. For dental work scale, I'd put those at about a 3, but my dentist is excellent (and he better be for that kind of money). More important, though, is to see what the differences will be in your dental health when you compare the likely outcomes of each type of procedure. Whichever is the more positive potential, I'd say do that. The dental pain should be relatively short lived when you think of how long you'll be living with your teeth, so always choose the option which gives you the better long-term health prosepects.
Whichever you choose, get it over with, start healing.